1. Field of the Invention
This invention relates to a storage control system and storage control method.
2. Description of the Related Art
Storage control systems such as RAID (Redundant Array of Independent Inexpensive Disks) system are known which comprise a plurality of physical data storage devices (for example, hard disk drives) arranged in an array. In a plurality of storage devices comprised by such a storage control system, one or a plurality of logical volumes, which are logical storage devices, are prepared. A storage control system is generally connected to a host device. Such a storage control system receives write requests from the host device and writes data received from the host device to a logical volume according to the write request, or receives read requests from the host device and transmits to the host device data read from a logical volume according to the read request.
In a computer system comprising a storage control system and a host device, when for example the data stored in the storage control system is backed up, a copy of a logical volume in which the original data is stored may be generated. In the generation of a copy volume in such cases, a technique employing “snapshots” is known, in which updated differential data is copied and stored within the storage control system, with the goals of both reducing the storage area employed for copying for cost reasons, and of shortening the time required when restoring an updated volume from a copy volume from the standpoint of improving restore operations. That is, such a storage control system may create a snapshot of a logical volume in order to manage the data configuration within the logical volume at a certain time (see for example Japanese Patent Laid-open No. 2001-216185).
There may be for example two types of logical volume, which are primary volumes and secondary volumes. Primary volumes and secondary volumes may be mutually associated (for example, put into a paired state). In snapshot-creation, for example, when the storage control system receives new data (hereafter called “update data”) from the host device and writes the data to a logical volume, the update data itself, or the original data which has been overwritten with update data, is written to a specific secondary volume (hereafter for convenience called a “differential volume”) as differential data. In taking a snapshot using such a method, each time update data is overwritten, the differential data is accumulated in the differential volume as an update history (or, old differential data may be overwritten by new differential data, and therefore lost). Consequently, from the start of creation of the snapshot until the snapshot is canceled (in other words, until all the differential data is discarded), the differential data is retained.
On the other hand, there are cases in which the host device may use the file system of the primary volume to manage data in the primary volume; in such cases, when for example discarding data in the primary volume, only changes to the file system are executed by the host device, and in the storage control system only the corresponding data in the file system is discarded. Consequently data which has become unnecessary for the host device may remain as differential data within the differential volume of the storage control system. Such cases are not limited to data discarding as described above, but can also occur upon data updates (for example, when updating large-size data with small-size data).